The present invention generally relates to sensors that determine the hydrocarbon content of aqueous liquids, and more particularly to a sensor for quantifying hydrocarbon content in aqueous liquids using both fluorescence spectral emissions and particulate size information derived from detection of optical scattering due to the interaction of oil droplets in the liquid and a coherent light signal.
Many industrial processes utilize an oil-content-monitor (OCM) to provide a real-time on-line measure of the amount of petroleum hydrocarbons present in process water or wastewater streams. Bilge discharge monitoring is a common example of OCM usage. Ships at sea treat bilge water to remove oily contaminants prior to discharging the bilge into the surrounding environment. Environmental regulations specify that bilge water may not be pumped overboard if the oil content exceeds 15 part-per-million (ppm) within the coastal zone, or 100 ppm at sea. Shipboard OCMs provide on-line measurements of the amount of fuel or oil present in the treated bilge water. The ship""s crew utilizes this information to make ongoing decisions as to whether the processed bilge may be lawfully discharged or requires further treatment. Examples of other OCM applications include on-line monitoring of: oil well process water discharge, car/aircraft wash facilities, power plant effluent, engine cooling water, desalination plant intake, boiler condensate, storm water runoff, and reclaimed groundwater.
Many existing OCM systems use optical methods to measure oil content. OCM sensors based on ultraviolet (UV) fluorescence, optical scattering, or optical transmission/absorption methods are common. Optical techniques have a xe2x80x9cstand-offxe2x80x9d advantage over other methods in that direct physical contact with the sample is unnecessary.
Most optically based OCMs are single-channel (zero order) instruments, i.e. they utilize one measured parameter to determine hydrocarbon content. The single parameter these instruments measure may include fluorescence emission at a single wavelength band, or suspended-particle scattering at a single angle, or optical absorption at a single wavelength band, or the ratio of single-angle scattering to single wavelength-band transmission, etc. Instrument calibration is performed by applying a mathematical transformation of the single measured datum in order to relate the raw signal to actual oil content. Single channel instruments offer the benefit of a simple univariate calibration model, e.g. the calibration is typically implemented as a linear function of system response.
Accurate quantification when the hydrocarbon species and matrix are not known a priori is simply not possible with single-channel (univariate calibration) methods. Single-channel (univariate calibration) instruments are adequate for applications where the hydrocarbon analyte, aqueous matrix, and mixing conditions are all well characterized and do not vary over time. However, as single-channel instruments they cannot provide accurate oil content measurements when any of the following conditions exist: a) when the type of hydrocarbon analyte is unknown or changing, b) when the background signal is varying, c) when matrix effects are present (i.e. when the sensitivity of the analyte is dependent upon the presence of other species), or d) when physical factors that effect emulsification, e.g. mechanical stirring, temperature, etc. vary. The inaccuracies are due to the fact that a single data point provides insufficient information to resolve multiple unknown parameters. If the instrumental sensitivity is significantly different for two or more types of petroleum products, for example diesel fuel and lube oil, and both are potentially present in the sample, then a given instrumental response cannot be uniquely associated with a single xe2x80x9coverallxe2x80x9d oil content. Single-channel instruments are also incapable of distinguishing between a signal arising from target analytes and background interference. Signal changes brought about by spectral or physical interferences, common in many applications, cannot be differentiated from signal changes arising from a change in oil content. In a dynamic environment, this leads to erroneous determinations of oil content.
Accurate measurement of small quantities of oil in water (e.g. low mg Lxe2x88x921) is extremely difficult when the hydrocarbon type and/or matrix is changing or when physical and chemical interferences are present. Because of the many parameters involved, consistently accurate measurement requires a multi-channel (multivariate measurement and calibration) approach. Multichannel fluorescence and multichannel light scattering can both be used to for real-time measurement of oil content in water. Each method has distinct advantages and disadvantages.
The fluorescence based methods detect the intensity of fluorescence emission from both dissolved and emulsified aromatic hydrocarbons (AHs) when irradiated with ultraviolet light. For a given sample, the wavelength dependent fluorescence emission generally varies with the specific wavelength band used to excite the sample. Excitation frequencies are usually in the 250-350 nm range. More than one excitation wavelength band may be used either simultaneously or in succession to generate multidimensional fluorescence emission spectra.
One of the attractive features of fluorescence is that it provides a very sensitive means of quantifying low levels of dissolved phase petroleum hydrocarbons (aromatics) in water. The aromatic constituents of petroleum, for example, benzene, naphthalene, and their derivatives, are many times more soluble in water than the non-aromatic components, and therefore constitute the bulk of the dissolved phase hydrocarbons in an aqueous mixture.
The response of a given fluorescence based OCM can vary significantly with the specific type of petroleum hydrocarbon analyte as well as with the size of emulsified oil droplets in the sample. The oil-type-dependent response is due to the fact that petroleum products have varying AH content as well as other varying constituents that may quench or absorb AH fluorescence. The AH dependent fluorescence response may vary with the crude or refined product type, (gasoline, lube oil, diesel fuel, jet fuel, etc), origin of crude stock, refining process, etc. In addition, the fluorescence response of oil-in-water emulsions also varies significantly with the size of the emulsified oil droplets. The dependence on droplet size can be complex. For a given oil (in water) volume, as the average droplet volume decreases the total number of droplets and total cross-sectional area increases potentially leading to higher fluorescence yield. However, changing droplet size may also affect solution equilibria and the partitioning of aromatic species into and out of solution. Hence the fluorescence response may increase at some wavelengths and decrease at others.
The light scattering methods generally measure 1) the attenuation of the intensity of light passing through a sample and/or 2) the light scattered by the sample at one or more angles. The scattering is due to the presence of emulsified droplets of oil in the sample. Multi-angle scattering can be used to estimate droplet (or particle) size distribution given in terms of droplet number per size-range, or volume concentration per size range. An advantage of scattering methods is that, unlike fluorescence that targets aromatic hydrocarbons exclusively, they are generally responsive to all types emulsified oils. A severe shortcoming, however, is that scattering cannot detect the dissolved phase of oil constituents. Scattering also poses difficulties in distinguishing between oil droplets and solid particles. Multi-angle scattering and re-emulsification methods help in distinguishing oil droplets from solid particles, but complete compensation for solids content is difficult.
The major technical challenge of OCM design is to maintain quantitative accuracy for online measuring of oil-in-water concentrations in the presence of unknown chemicals and physical interferences, including fluorescent organic compounds, detergents, suspended solid particles, and dissolved salts. Existing instruments and methods for calibrating their response are not appropriate for complex systems. Therefore, a need exists for a reliable and accurate method for determining the concentration of oil droplets in aqueous media.
The present invention provides an oil content monitor (OCM) that combines the use of multichannel fluorescence and droplet size information to quantify oil contamination of an aqueous media. Fluorescence emission intensity is measured at multiple wavelengths. The intensity at a given wavelength is a function of oil content. The relative intensity differences at different wavelengths are a function of the analyte itself, and can be used to determine which species or class of oil is present. The oil droplet size distribution and oil volume concentration is estimated from optical scattering measurements made at multiple small angles. The invention includes a sample cell; a first light source for generating a first light signal that stimulates fluorescent emissions when the first light signal irradiates hydrocarbons present in an aqueous solution in the sample cell; a spectral detector for generating a first electrical signal that represents spectral components of the fluorescent emissions in response to detecting the fluorescent emissions that are emitted from the sample cell; a second light source for generating a coherent light signal that is transformed into scattered light signals when the coherent light irradiates oil droplets in the aqueous solution; a detector for generating a second electrical signal in response to detecting scattered light signals emitted from the sample cell, where the second electrical signal represents the intensities and scatter angles of the scattered light signals; and a processor for determining a particle size distribution of the oil droplets from the second electrical signal and the hydrocarbon content of the aqueous solution from the first electrical signal and the particle size distribution.
Another embodiment of the invention uses a single optical energy source and includes a sample cell. A light source generates a coherent ultraviolet light signal that stimulates fluorescent emissions when the first light signal irradiates hydrocarbons present in an aqueous sample in the sample cell, where the coherent ultraviolet light signal is transformed into scattered light signals when the coherent ultraviolet light irradiates oil droplets in the aqueous sample. A spectral detector generates a first electrical signal that represents spectral components of the fluorescent emissions in response to detecting the fluorescent emissions that are emitted from the sample cell. The spectral detector also generates a second electrical signal in response to detecting scattered light signals emitted from the tube, where the second electrical signal represents the intensities and scatter angles of the scattered light signals. A processor determines a particle size distribution of the oil droplets from the second electrical signal and estimates the hydrocarbon concentration content of the aqueous sample from the first electrical signal and the particle size distribution.
An important advantage of the invention is that it employs both the spectral components of fluorescent emissions generated by irradiating the aqueous test sample with ultraviolet light, and the particle size distribution of oil particles suspended in the aqueous test sample in order to determine the oil content of the aqueous test sample.
Other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims.